1. Field of the Invention
The present invention relates to dielectric filters, dielectric duplexers, and communication devices which are mainly used in the microwave band.
2. Description of the Related Art
A related dielectric filter using a dielectric block substantially in the form of a rectangular solid is described with reference to the perspective view of FIG. 9.
A pair of through holes 2a, 2b are formed in the dielectric block. Internal conductors 3a and 3b are formed on the internal surfaces of the through holes 2a, 2b to form resonator cavities. An external conductor 5 is formed on the outer surface of the dielectric block 1. Furthermore, conductor-free portions 4a and 4b are provided in the vicinity of one open end surface of each of the resonator cavities to form open ends. The other ends of the resonator cavities are short-circuited to the external conductor 5. Input-output electrodes 6a and 6b are formed so as to be coupled with the open ends of the resonator cavities. The outer periphery of the input-output electrodes are defined by conductor-free portions 7a and 7b that extend from the mounting surface of the dielectric block to the opposite end surfaces which extend parallel to the linear direction of the resonator cavities thereof. A complete dielectric filter is thus constructed.
However, in the related dielectric filter using such a dielectric block, the following problem occurs.
In the related dielectric filter, the input-output electrodes 6a and 6b and the conductor-free portions 7a and 7b are formed with square shapes which extend from the mounting surface to the side surfaces. That is, the outer shapes of the input-output electrodes 6a and 6b and the external-conductor-free portions 7a and 7b are formed by lines meeting at right angles.
As a result, as shown in FIG. 10, current is concentrated in the vicinity of the input-output electrodes 6a and 6b and the conductor-free portions 7a and 7b, and accordingly conductor loss is high. Therefore, the unloaded Q factor of the dielectric filter deteriorates and the insertion loss and attenuation characteristics worsen.
It is an object of the present invention to construct a dielectric filter and dielectric duplexer using a dielectric block having reduced conductor loss caused by concentration of a current in the external conductor of the dielectric block and having a high unloaded Q factor, and a communication device using the dielectric filter and dielectric duplexer.
The invention includes a dielectric filter, comprising:
a dielectric block having a plurality of outer surfaces including opposed top and bottom surfaces, opposed side surfaces and a mounting surface, the dielectric block also having a plurality of through holes extending between the opposed top and bottom surfaces;
respective internal electrodes located on internal surfaces of the through holes;
an outer conductor formed on at least some of the outer surfaces; and
first and second input-output electrodes formed on the outer surfaces, the first input-output electrode extending from the mounting surface to one of the side surfaces, the second input-output electrode extending from the mounting surface to the other of the side surfaces, each of the first and second input-output electrodes being separated from the outer conductor by a respective conductor-free portion wherein at least one corner portion of an outer shape of each of the conductor-free portions is formed as a continuous line having no right angles.
Furthermore, according to a preferred embodiment of the present invention, a dielectric filter is constructed such that at least one corner portion of each of the outer shape and inner shape of the external-conductor-free portions is formed as a continuous line having no right angle.
Furthermore, according to the present invention, a dielectric duplexer is constructed by using the dielectric filter.
Furthermore, according to the present invention, a communication device is constructed by using the dielectric filter or dielectric duplexer.